Measure stuff with your PSP, without any ruler! Lay the thing you want to messure on your PSP Display after you’ve started OnScreenMeasure. Then move the dot to the start of the thing and press X. Then move the second dot to the end of the thing and press START to measure. The length will be given in cm! Press SELECT to restart or correct the second dot with the arrow keys.

http://forums.qj.net/psp-development-forum/162822-onscreenmessure-v0-3-release.html